The fierce scrutiny of bank misconduct is eating into the profits of National Australia Bank, which said it would take extra provisions in the second half for "regulatory compliance investigations."

NAB flagged the extra provisions - money set aside to cover a liability - in a trading update on Tuesday, which said unaudited cash profit was 3 per cent lower in annual terms in the June quarter, at $1.65 billion.

NAB's bottomline result dipped because of higher spending on investment and credit impairment charges.Credit:Pat Scala

"As we make progress towards resolving several previously disclosed regulatory compliance investigations, we expect to recognise additional provisions in the 2H18 result, noting there are significant uncertainties in determining a provisioning outcome at this time," he said in a statement.

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NAB did not say how much it would set aside in extra provisions, but referred to contingent liabilities in its most recently half year, which included a wide range of potential costs. These included its compensation of clients who were charged "fees for no service," an issue that has been investigated by the royal commission over the past week - and a "wealth advice review" being carried out by the Australian Securities and Investments Commission.

The contingent liabilities also list a program to improve the bank's compliance with anti-money laundering and counter-terrorism finance laws.

"The royal commission is challenging us with its focus on where we have let customers down. We are determined to respond and become a better bank through living our purpose and values every day," Mr Thorburn said in a statement.

The higher provisioning costs would be excluded from its previous guidance for expense growth of 5 to 8 per cent.

NAB's update included data on the recent damage to banks' corporate reputations. Its net promoter score - a measure of how likely customers are to recommend the bank - declined from -9 to -14 in the three months to June, which NAB said was an industry-wide trend.

The trading update also said charges for impaired loans increased by 9 per cent compared with a year ago, but the overall share of customers who were more than 90 days behind on their loan repayments was steady at 0.71 per cent of gross assets.

Meanwhile, ANZ also released an update on its loan book, which showed its loan growth had slowed to well below the industry average in the latest June quarter.

It said there had been a "meaningful" fall in average maximum borrowing capacity in the home loan market, due to regulatory changes and tighter credit policies by banks. It said ANZ's provisions for bad debts were the lowest quarterly charge since 2014. ANZ shares rose 1.3 per cent to $29.55 in morning trade.